Question #2: The Sun's Fuel

Robert asks:

"I know this sounds like a stupid question, but where does the Sun store its fuel to sustain the fusion process if the fuel is mixed in with all the explosions? When I hear that the Sun has enough fuel to continue for another 4 or 5 billion years, I picture a partition in which the fuel is slowly metered out. I have watched dozens of shows about the Sun and none of them seemsto address where the fuel is stored. Please enlighten me on this."

Everyday Einstein: That's actually a great question, Robert! The hydrogen fuel actually sits at the core of the star. The hydrogen atoms bounce around in there until they slam against each other with enough force to cause fusion, turning the hydrogen into helium. Energy is released in this process and travels outwards in the form of light, heat, and other assorted rays (x-rays, gamma rays, etc.)

However the gravity of the core, combined with the pressure of the outer layers of the sun pushing down on the core, cause almost all of the hydrogen and helium to stay in the core.

Assuming the Sun follows the same route as similar stars, eventually, the core will start to run out of hydrogen. With fewer collisions resulting in less energy being produced, the pressure inside the core will decrease, allowing the outer layers to compress. There are a few other things that happen at this point, but eventually the collapsing outer layers cause the pressure and temperature of the core to increase enough that the Sun will start using helium for fusion, creating heavier elements like carbon.

This process repeats a couple of times, resulting in heavier and heavier elements being created in the core, until the star eventually dies. Here's a great article that explains it in more detail.

Question #3: Pucker Up

Finally, Adam wrote in to ask the following:

"Dr. Falin, I have always wondered about this: Why is it that the air appears to be cold when you blow with your lips puckered but it appears to be warm when you blow with your mouth only slightly more open?"

Everyday Einstein: It's funny, I actually had never noticed that until you mentioned it, Adam. After some highly scientific research and a few experiments involving repeated hyperventilating and some drinking straws, I think the answer is that either way you exhale (puckering or otherwise), the air coming out of your mouth is in fact the same temperature. However when the faster air comes out of your mouth via puckering, its higher velocity allows it to sweep in the surrounding air along its way. Since the surrounding air is (usually) cooler than your internal body temperature, the air feels cooler.

Here are a couple of experiments you can try to see this in action:

1. Do the pucker/wide mouth comparison again, but hold your finger as close to your lips as as possible. This minimizes the amount of outside air that can be swept up by the higher velocity of puckered breath. You should notice that the temperature is significantly warmer than when you do the same thing with your finger far away.

2. Try blowing through a drinking straw, again with your finger close to the straw’s opening. Even though you’re still using higher velocity air, the breath should feel warm.

This is a hydrodynamic phenomenon, (meaning it involves the forces of liquids pushing on each other), called entrainment. When one fluid (be it gas or water) sweeps another fluid along in its flow, we say the first fluid has entrained the second one. If you decide to research this further, I’ll warn you that nearly every branch of science uses the word “entrainment” slightly differently.